Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. The automotive industry expends significant resources in the development of hydrogen fuel cells as a source of power for vehicles. Fuel cell vehicles are more efficient and generate fewer emissions than today's vehicles employing internal combustion engines.
A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen-rich gas or pure hydrogen and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode. The work may be used to operate a vehicle, for example.
Proton exchange membrane (PEM) fuel cells are popular for vehicle applications. The PEM fuel cell generally includes a solid-polymer-electrolyte proton-conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture, and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation. These conditions include proper water management and humidification, and control of catalyst poisoning constituents, such as carbon monoxide (CO).
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For the automotive fuel cell stack mentioned above, the stack may include about two hundred or more bipolar plates. The fuel cell stack receives a cathode reactant gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include liquid water as a stack by-product. The fuel cell stack also receives an anode hydrogen reactant gas that flows into the anode side of the stack.
The fuel cell stack includes a series of flow field or bipolar plates positioned between the several MEAs in the stack. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode gas to flow to the anode side of the MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode gas to flow to the cathode side of the MEA. The bipolar plates may also include flow channels for a cooling fluid.
The bipolar plates are typically made of a conductive material, such as stainless steel, titanium, aluminum, polymeric carbon composites, etc., so that they conduct the electricity generated by the fuel cells from one cell to the next cell and out of the stack. Metal bipolar plates typically have a natural oxide on their outer surface that makes them resistant to corrosion. However, this oxide layer is not conductive, and thus increases the internal resistance of the fuel cell, reducing its electrical performance.
As is well understood in the art, typically the membranes within a fuel cell need to have a certain relative humidity so that the ionic resistance across the membrane is low enough to effectively conduct protons. During operation of the fuel cell, moisture from the MEAs and external humidification may enter the anode and cathode flow channels. At low cell power demands, typically below 0.2 A/cm2, water accumulates within the flow channels because the flow rate of the reactant gas is too low to force the water out of the channels. As the water accumulates, it forms droplets that continue to expand because of the hydrophobic nature of the plate material. The contact angle of the water droplets is generally about 90° in that the droplets form in the flow channels substantially perpendicular to the flow of the reactant gas. As the size of the droplets increases, the flow channel is closed off, and the reactant gas is diverted to other flow channels because the channels flow in parallel between common inlet and outlet manifolds. Because the reactant gas may not flow through a channel that is blocked with water, the reactant gas cannot force the water out of the channel. Those areas of the membrane that do not receive reactant gas as a result of the channel being blocked will not generate electricity, thus resulting in a non-homogenous current distribution and reducing the overall efficiency of the fuel cell. As more and more flow channels are blocked by water, the electricity produced by the fuel cell decreases, where a cell voltage potential less than 200 mV is considered a cell failure. Because the fuel cells are electrically coupled in series, if one of the fuel cells stops performing, the entire fuel cell stack may stop performing.
In a PEM fuel cell environment, the bipolar plates are in constant contact with highly acidic solutions (pH 3-5) containing F−, SO4−, SO3−, HSO4−, CO3−, and HCO3−, etc. Moreover, the cathode operates in a highly oxidizing environment, being polarized to a maximum of about +1 V (versus the normal hydrogen electrode) while being exposed to pressurized air. Finally, the anode is constantly exposed to atmospheric hydrogen. Hence, contact elements made from metal must be resistant to acids, oxidation, and hydrogen embrittlement in the fuel cell environment. As few metals exist that meet this criteria, contact elements have often been fabricated from large pieces of graphite which is corrosion-resistant, and electrically conductive in the PEM fuel cell environment. However, graphite is quite fragile, and quite porous making it extremely difficult to make very thin gas impervious plates therefrom.
Lightweight metals, such as aluminum and its alloys, have also been proposed for use in making fuel cell contact elements. Such metals are more conductive than graphite and can be formed into very thin plates. However, such light weight metals are susceptible to corrosion in the hostile PEM fuel cell environment. In light of the corrosion sensitivity of lightweight metals, efforts have been made to develop protective coatings. Yet some of these protection methods increase the electrical resistance of the aluminum plate to unacceptable levels. Other methods of protection keep the conductivity at an acceptable level, but do not sufficiently achieve the desired level of protection.